This invention relates to the field of lasers and, more particularly, to reflectors for pulsed lasers.
The objective of pulse lasers is to deliver energy from a flashlamp to a particular system. A linear, gas-filled, electric discharge flashlamp, e.g. flashlamp 10 in FIG. 1, emits light rays 20 uniformly around the flashlamp circumference. The radiating gas plasma 15 is optically thick, meaning that the light emanates from the surface of the plasma and not from within its bulk. In this way, the plasma approximates a black-body source. Flashlamps in practical use also require a coolant which is usually contained in a jacket surrounding the flashlamp.
Flashlamps are used as optical "pumps" for many types of lasers. Examples include solid state lasers, such as Nd.sup.2++ YAG and Nd.sup.2++ glass, liquid lasers, such as organic dye lasers, and gas lasers, such as pulsed chemical lasers. Generally speaking, the efficiency of the laser device depends upon the collection efficiency of the reflector used to couple the flashlamp light to the laser material in the gain volume. Gain volume is the volume of space occupied by the laser material from which the laser light emanates.
Reflectors are used in flashlamp-pumped pulsed lasers to direct the uniformly distributed flashlamp rays toward the laser materials in the gain volume. The optimal reflector surface should direct all the flashlamp rays to the laser materials through a minimum aperture. The reflector surface should not allow those rays to reenter the flashlamp plasma.
Two commonly-used reflectors are imaging reflectors and diffuse reflectors. An example of an imaging reflector is the double ellipse reflector 30 shown in FIG. 2. Imaging reflector 30, however, has several disadvantages. Light which enters regions 40 and 45 is not imaged at the correct location, and thus is not used efficiently. Instead, that light is dispersed throughout the reflector cavity (region 40) or is reflected back into flashlamps 50 and 51 (regions 45 and 46, respectively) and reabsorbed. Reabsorbed light is not used efficiently since it does not merely "repump" the flashlamp. There is a large fraction of such light in the ultraviolet region which is absorbed by the lamp wall and lost.
An example of a diffuse reflector 60 is shown in FIG. 3. Reflector 60 relies on multiple reflections from a diffuse surface to allow light to emerge from aperture 65. Reflector 60 is also quite lossy due both to the less than 100% reflection and the reabsorption of light by flashlamp 70.
The problems with reflector design often prevent certain lasers from having practical application. Flashlamp pumped dye lasers, in the past, have not been considered viable for many applications because of poor laser efficiency and short flashlamp lifetime. Power requirements, system and subsystem performance and, therefore lifetime, depend directly on laser efficiency. Increased laser efficiency relaxes the power and energy requirements on a flashlamp and increases the life of the lamp. In flashlamp pumped dye lasers, the laser efficiency varies directly with the fraction of light emitted by the lamp and deposited in the gain volume, and varies inversely with gain volume.
One attempt to improve flashlamp reflector design is explained in M. R. Siegrist, "Cusp shaped reflectors to pump disk or slab lasers," Applied Optics 2167, Vol. 15, No. 9 (September 1976). The reflector surface proposed in that article has a cross-section which is a curve defined as a simple involute, as shown in FIG. 4. In a rectangular coordinate system with the center of flashlamp 80 being the origin, reflector 75 has a curved reflector surface whose cross-section defines a curve: EQU x=.+-.(r cos .theta.+r.theta. sin .theta.),
and EQU (0.ltoreq..theta..ltoreq..pi.) EQU y=r sin .theta.-r.theta. cos .theta.,
where
.theta. is an angular parameter, and PA1 r is the radius of the entire flashlamp and surrounding cooling jacket.
This simple involute reflector does not possess a high efficiency, since it does not account for the angles at which light rays leave an actual flashlamp. In addition, the ideal involute reflector has a very wide aperture 85 equal to 2.pi.r. [definition for reflector aperture] reflector aperture is the opening from which light emanates from the reflector/lamp assembly.
In the flashlamp shown in FIG. 4, r=r.sub.f +(w.sub.j +w.sub.c), where r.sub.f is the diameter of the flashlamp 80, w.sub.j is the width of the cooling jacket 92 and w.sub.c is the width of the coolant contained by the jacket 92. The coolant is necessary to prevent the flashlamp from overheating. A typical value for r.sub.f is 3 mm, a typical value for w.sub.j is 1 mm and a typical value for w.sub.c is 2 mm. Thus, r is typically about 6 mm, so a conventional simple involute reflector 75 typically has a span of about 37.7 mm.
For efficiency, flashlamp reflector designs should maximize the pumping density (joules per unit volume). The wide aperture of the simple involute reflector reduces that density since it increases the gain volume. A flashlamp reflector should also maximize efficiency and pulse energy simultaneously. Other conventional reflectors that add favorable efficiency are scalable to larger pulse energies only through extending the gain length.
One objective of this invention is to improve reflectors for flashlamp lasers such that the reflectors direct the flashlamp energy through a minimum aperture and away from the flashlamp.
Another objective is to provide an optimal reflector surface configuration which can easily be determined for any flashlamp laser.